Field
This invention relates generally to an air boundary layer diverter for an aircraft and, more particularly, to a swept gradient air boundary layer diverter for a supersonic aircraft that includes a V-shaped swept expansion and compression ramp and diverter trough positioned in front of an engine inlet of the aircraft.
Discussion
Some modern aircraft must have the capability to operate at supersonic speeds, i.e., above Mach 1, which requires the aircraft to be highly aerodynamic and relatively low weight. In order to operate at supersonic speeds, the engines of such aircraft require a relatively large air inlet, where a typical air inlet for a supersonic aircraft will include a specially shaped leading edge, sometimes referred to as a cowl, and/or compression ramps that reduce the speed of the airflow into the engine to be suitable for proper operation of the engine. The design of these types of air inlets is challenging for aerodynamic operation.
For an aircraft in free flight, a low velocity, low pressure boundary layer of air builds up on the fuselage of the aircraft. The air boundary layer is generated as a result of friction forces on the aircraft fuselage, where air immediately adjacent to the fuselage has a zero velocity and as the distance from the fuselage increases, the velocity of the air also increases as determined by the speed of the aircraft. As the distance from the fuselage increases, the pressure forces of the airflow overcome the friction effect of the fuselage, where at some distance from the aircraft, the airflow becomes a free stream. If this low speed air boundary layer is ingested into the engine air inlet, the engine can encounter operability issues, such as a surge or stall, due to distortion levels beyond the engine's limitations, or rotating machinery high cycle fatigue issues due to increased distortion levels. Ingested boundary layer airflow also reduces the engine thrust and efficiency, which results in a reduced speed of aircraft operation.
In order to overcome these issues caused by the air boundary layer, it is known in the art to design supersonic aircraft with an air boundary layer diverter that prevents the boundary layer air from entering the engine air inlet. A traditional air boundary layer diverter on a supersonic aircraft includes a slot formed between the aircraft fuselage and the air inlet through which the boundary layer air flows, instead of flowing into the inlet. The width of the slot is selectively designed so that the inside edge of the cowl is at a location where only the free air steam is occurring. Such air boundary layer diverters have been shown to be effective in preventing the boundary layer air from entering the air inlet, but they reduce aircraft performance as a result of having a larger aircraft cross-sectional area that increases aircraft drag. Further, the airflow of the boundary layer is directed around the cowl, which often causes this air to impact various structures that are required to incorporate the slot diverter, which also increases drag.